This invention relates generally to the fabrication of orthodontic and medical instruments and devices and components thereof, and, more particularly to the fabrication thereof from a specially processed Nickel-Titanium shape memory alloy.
The concept of using shape memory alloys for eyeglass components has been suggested in numerous articles and patents, and the application of these alloys for medical use is well advanced.
Andresson in U.S. Pat. No. 4,037,324 suggested the use of shape memory alloys for orthodontic arch wires, and since this early patent many other patents have issued claiming the advantages of using shape memory alloys for both orthodontic as well as medical components.
The driving force for making metal medical devices from shape memory alloys lies in their great resistance to permanent deformation as compared to conventional alloys employed in this application. Alloys used in conventional orthodontic arch wires and various medical instruments have relied on stainless steel, complex high nickel alloys such as Elgiloy(trademark) and titanium based alloys, all of which can be given quite high yield strength through work hardening, but in use can be fairly easily permanently deformed. Normal metals, even with very high yield strength, cannot sustain strains much greater than 0.2% without suffering a permanent set. Once a bend or kink has been sustained in a medical instrument or device fabricated from one of the above conventional alloys it is virtually impossible to remove. The unusual property of pseudoelasticity exhibited by shape memory alloys such as Auxe2x80x94Cd, Cuxe2x80x94Znxe2x80x94Al, Nixe2x80x94Ti and many others makes possible the complete xe2x80x9celasticxe2x80x9d recovery of strains as great as 10%. Due to its high recoverable strain and its excellent resistance to corrosion, the shape memory alloy of preference for orthodontic and medical components has been within the Nixe2x80x94Ti family of alloys.
Shape memory alloys belong to a class which exhibit what is termed thermoelastic martensite transformation. The term martensite refers to the crystalline phase which is produced in steels when quenched from a high temperature. The phase which exists at the elevated temperature is referred to as austenite; these terms have been carried over to describe the transformations which occur in shape memory alloys. When a steel has been quenched from the austenitic temperature to martensite, to again form austenite requires heating the structure to quite high temperatures, usually in excess of 1400xc2x0 F.
By contrast, the thermoelastic shape memory alloys can change from martensite to austenite and back again on heating and cooling over a very small temperature range, typically from 18 to 55xc2x0 F. The transformation of a shape memory alloy is usually described by its hysteresis curve, FIG. 1. In this figure it is shown that on cooling from the austenitic phase, often called the parent phase, martensite starts to form at a temperature designated as MS and upon reaching the lower temperature, MF the alloy is completely martensitic. Upon heating from below the MF temperature the martensite starts to revert to the austenitic structure at AS and when the temperature designated as Af is reached the alloy is completely austenitic. These two phases or crystalline structures have very different mechanical properties: the Young""s Modulus of austenite is xcx9c12xc3x97106 psi while that for martensite is xcx9c4xc3x97106 psi. and the yield strength, which depends on the amount of cold work the alloy is given, ranges from 28 to 100 ksi for austenite and from 10 to 20 ksi for martensite.
The unique feature of shape memory alloys is their ability to recover deformation. When a shape memory alloy specimen, hereinafter referred to as SMA, in its martensitic form is subjected to stress, the strain is accommodated by the growth and shrinkage of individual martensite variants rather than by the mechanisms which prevail in conventional alloys: slip, grain boundary sliding and dislocation motion. When deformed martensite is heated to the austenite finish temperature AF the part reverts to its original undeformed state. This process is illustrated in FIG. 2.
Although this process could be utilized in medical devices to recover accidental bending and kinking, the mechanical properties of martensite, its yield strength and its modulus of elasticity, are too low for this application, and, in addition, heating medical devices is not a convenient process. Fortunately, another mode of deformation of SMAs provides the properties and behavior ideally suited to this service; this is pseudoelastic behavior.
As indicated above, martensite forms when a SMA is cooled from the austenitic region to below the MS temperature; it can also form when the austenite is stressed to above some critical level. The martensite so formed is called stress-induced-martensite or SIM. Since the martensite formed under stress is at a temperature where it is not stable, when the stress is removed the alloy spontaneously reverts to its prior unstressed shape. This behavior is illustrated in FIG. 3. It can be observed that the reversion stress is lower than the stress at which martensite forms. These stresses are referred to as the upper and lower plateau stresses and their magnitude is dependent on the thermal and mechanical treatment which the SMA has received. As the temperature of the specimen is raised, the stress magnitude required to produce SIM is increased, as shown in FIG. 4, however when the specimen reaches a critical temperature above AF, designated as MD, stress induced martensite cannot be formed, no matter how high the stress. This behavior gives rise to a limitation on using the pseudoelastic property in many situations since it places a limit on the temperature range over which pseudoelasticity is observed; typically in the NiTi alloys, this is a temperature range of about 60xc2x0 C. (108xc2x0 F.), although a 40xc2x0 C. (72xc2x0 F.) range is more typical. The desirable temperature range for medical and orthodontic application is in the region of body temperature, +40xc2x0 C.xc2x110xc2x0 C., readily achieved in these alloys.
Prior practitioners of the art of applying SMAs to medical and orthodontic components have resorted to the use of an SMA which has been cold worked in the martensitic state followed by a low temperature anneal to give a combination of shape memory behavior and superelastic characteristics. This processing gives a component with an elastic range of approximately 3% over a temperature range of xe2x88x9220 to +40xc2x0 C. We have found that by using an alloy with higher than the equiatomic Ni/Ti ratio, subjecting it to a high temperature annealing followed by water quenching and a subsequent aging treatment, that we obtain a pseudoelastic behavior combined with excellent forming characteristics and a strain recovery of at least 3% over a temperature range from xe2x88x9220 to +40xc2x0 C. The treated alloy yield strength ranges from 42 to 72 Ksi.
An object of the present invention is to provide a nickel-titanium alloy which is particularly useful for medical instruments and devices, as well as components thereof.
Another object of the present invention is to provide an alloy having pseudo-elastic properties and which is useful for medical instruments and devices, as well as components thereof.
A further object of the present invention is to provide a material for making medical instruments and devices as well as components thereof which are formable without the creation of cracks.
These and other objects of the present invention are accomplished by providing a nickel-titanium shape memory alloy which is especially useful in making medical instruments and devices, as well as components thereof and has desired pseudoelastic properties, characterized by:
allowing large plastic deformations during fabrication of the part before the desired pseudoelastic properties are established,
having pseudoelastic properties without using cold working,
having greater than 2.5% elasticity over the temperature range where these devices are usually located, and
being capable of undergoing large amounts of cold or hot forming without danger of cracking/fracturing during the forming operations required to make the part.
The unusual property of pseudoelasticity exhibited by shape memory alloys such as Auxe2x80x94Cd, Cuxe2x80x94Znxe2x80x94Al, Nixe2x80x94Ti and many others makes possible the complete xe2x80x9celasticxe2x80x9d recovery of strains as great as 105. Due to its high recoverable strain and its excellent resistance to corrosion, the shape memory alloy of preference for medical instruments and devices, as well as components thereof has been within the Nixe2x80x94Ti family of alloys.
The requirement of forming a medical instrument or devices, as well as components thereof from a piece of SMA wire or strip and controlling the amount of cold work it receives, both initially and in the final steps of component fabrication, followed by an annealing step which may require several hours, is considerably more complicated than the method of the present invention.
Prior practitioners of the art of applying SMAs to medical instruments and devices, as well as components thereof have recognised the temperature limitations discussed above and have resorted to the use of an SMA which has been cold worked in the martensitic state followed by a low temperature anneal to give a combination of shape memory behaviour and superelastic characteristics. This processing gives a component with an elastic range of approximately 3% over a temperature range of xe2x88x9220 to +40xc2x0 C.
Nickel-titanium alloys rendered pseudoelastic by a combination of cold work and heat treatment have a high yield strength which must be reduced by an annealing treatment requiring long periods of time to arrive at a satisfactory yield strength for medical instrument and device service. If the starting material for forming the medical component has already been cold work then subsequent forging or forming of the part may result in breakage.
In pseudoelastic behaviour arising out of SIM, the upper plateau stress in this process can be changed by a combination of cold work following by an annealing treatment. Another form of superelastic behaviour is obtained when a shape memory alloy in the martensitic state is cold worked, yielding a material with the low modulus characteristic of martensite but with complete elastic behaviour up to a 4% strain. In addition, this behaviour is observed over a temperature range of from xe2x88x92200 to +150xc2x0 C.
Past experiments on the precipitation hardening process, for instance by Nishida et al, Scripta Met, Vol 18, pp1299-1302, 1984, show that there is an optimum aging temperature to achieve the fine precipitates needed to increase austenite strengthening. Austenite yield strength must be high in order to have SIM proceed without having slip deformation of the matrix and permanent strain. A range of solution treatments and aging times and temperatures have been studied and reported in the literature for nickel titanium alloys.
The treatment and the alloy selection provided by the present invention is a modification of those commonly proposed. Prior studies have not provided detailed information on the temperature range over which the pseudoelastic behaviour is observed in alloys subjected to solution treatment and aging. With the treatment described, the present invention provides a method of producing a pseudoelastic nickel-titanium alloy which exhibits properties ideal for easy fabrication of medical instruments and devices as well as components thereof combined with those properties desired for a medical component which features high resistance to accidental damage.
The present invention provides using an alloy with higher than the equiatomic Ni/Ti ratio, subjecting it to a high temperature solution treatment at above 750xc2x0 C., followed by water quenching, and a subsequent aging treatment, that a pseudoelastic behaviour is obtained combined with excellent forming characteristics and a strain recovery of at least 3% over a temperature range from xe2x88x9220 to +40xc2x0 C. The treated alloy yield strength ranges from 42 to 72 Ksi.
One process to obtain pseudoelastic behaviour is by a solution heat treatment of a high nickel SMA at about 850xc2x0 C. followed by water quenching and then precipitation hardening at a lower temperature. High nickel alloy means alloys with a nickel content in excess of 50.5 atomic %.
The present invention seeks to provide a shape memory alloy and process which reduces the complexity of producing components for medical devices by using a precipitation hardening treatment of a high nickel alloy rather than the presently used cold working and heat treating. The resulting components are characterised by pseudoelastic properties which dramatically reduce the chance for accidental deformation or kinking. The precipitation process combined with the particular nickel-titanium alloy composition employed features a relative low upper plateau stress which renders the components flexible which, in turn, make medical components fabricated in the described manner easy to use.
By contrast with the prior art, forming the medical components when the alloy of this invention has been solution treated is quite easy, since in this condition it has excellent ductility. After forming, the component is subjected to an aging treatment which gives the part the pseudoelastic properties desired in medical components.
Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which: